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Interne t-To-Go, Now With Mobile IP

By Peter Rysavy Two trends in the computing industry are converging. One is the expanding scope of the Internet. The other is the surge in use of mobile computers for telecommuters and other remote users. These workers need access to IP-based networks--both corporate intranets and the Internet at large. A new Internet standard, Mobile IP, will help keep mobile computers seamlessly connected, independent of location. And Mobile IP supports any underlying medium--wireless or wired.

Using distributed routing tables, Internet routers pass on IP datagrams based on IP address, part of which specifies the destination computer's subnet. This addressing scheme, however, prevents a computer with a fixed IP address from connecting to a different subnet. The simplest way around this is to not assign fixed IP addresses, and instead, let computers obtain an IP address dynamically by using DHCP, for examp le. This approach also simplifies the management of IP addresses.

But a temporary address has two major limitations: Other nodes cannot easily originate transmissions to the mobile node because they cannot effortlessly learn its new IP address, and each time a mobile computer obtains a new IP address, Internet client software applications must be restarted. If your TCP/IP stack engages DHCP only at startup, you'll have to restart it as well. Moreover, temporary addresses do not let you roam seamlessly from one IP subnet to another. This is a handicap especially for wireless LANs--a doctor with a wireless LAN connection needs to maintain IP connections throughout the hospital as he or she accesses patient records.

Mobile IP was invented to let hosts with a fixed IP address connect to any IP subnet and immediately be reachable from the Internet.

How It Works Mobile IP, an extension to IP, is a recent Internet standard specified in RFC 2002, "IP Mobility Supp ort." The idea behind it is concept ually simple, though a number of complications may arise when using it. Mobile IP consists of three components: the mobile node, a home agent and a foreign agent. The mobile node is built into a TCP/IP stack or can exist as a "shim" under a TCP/IP stack. The home agent operates on a router or a workstation on the mobile node's home subnet. The foreign agent operates on a router or workstation on a foreign network where the mobile node is visiting, or on the mobile node itself under certain conditions. The beauty of Mobile IP is that these are the only elements required. No other changes are needed in any other part of the Internet, including routers or other systems, such as DNS.

When a mobile node comes up on a network, it first determines whether it is on its home network or on a foreign network. It does so by listening for a local broadcast message from a home agent or foreign agent. Alternatively, it can solicit an agent advertisement message. These initial and subsequent registration messages are base d on extensions to Internet Control Message Protocol (ICMP) Router Discovery specified by RFC 1256.

When the mobile node is on its home subnet--the one specified by its IP address--the mobile node informs the home agent of its presence. From there, IP addressing and datagram delivery work as they would without Mobile IP. The situation changes when the mobile node connects to a foreign network. There it obtains a "care of address," which is the foreign agent's IP address. The mobile node registers with its home agent and gives the home agent its care of address. Alternatively, if DHCP is available on the foreign network, the mobile node can obtain a temporary address, register this with the home agent and act as its own foreign agent.

Once the mobile node has registered with the home agent, IP traffic addressed to the mobile node is received by the home agent, encapsulated in another IP datagram and then "tunneled" to the foreign agent. The foreign agent forwards the datag rams to the mobile node. Two fo rms of encapsulation are specified in related standards RFC 2003 (IP Encapsulation within IP) and 2004 (Minimal Encapsulation within IP).

In the reverse direction, the mobile node can bypass the home agent and send datagrams directly to their destination (see "Triangular Routing of Data,"). This results in a triangular routing of traffic, which is not necessarily efficient but is effective. In addition, when a mobile node changes its location, it can register with a new foreign agent, though traffic directed by the home agent to the "old" foreign agent will be lost until the new mobile node has registered its location.